Optical interferometer for detection of small displacements

ABSTRACT

An optical interferometer for detecting small displacements. Two mirrors form a Fabry-Perot cavity. One mirror is mounted to be subject to movement by external displacement forces. The other mirror is subjected to feedback control to reduce low-frequency variations in length of the laser cavity.

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FOREIGN PATENTS OR AlPlPLlCATllONS Inventor figfiff sawmillsavgny'sur'orge* 1,068,054 5/1967 Great Britain ..356/112 {73] Assignee:tComnngnie Genernlle d'mectrlcite, Paris, OTHER PUBLICATHONS 1F TameBruce, C. F. On Automatic Parallelism Control in a [22] m; m 19%Scanning Fabrey-lerot Interferometer." Applied Optics,

Vol.5 No. 9, Sept. 1966, p. 1447- 1452. [21] Appl. No; 775/1169 PrimaryExaminerRonald L. Wibert 52 0.0. c1. "350/112, 250/199 AssistantExaminer-T Major 51 111111.101. ..c011 9 02 Alwmewsughrw, Rothwell,Mion, Ziml & Macpeak 511 11 11111 mom-1:11 ..356/106,112;331/94.5;

179/121; 250/199 W TRAC'H An optical interferometer for detecting smalldisplacements. [56] mdhmmes (med Two mirrors form a lFabry-Perot cavity.One mirror is UNITED STATES PATENTS mounted to be subject to movement byexternal displacement forces. The other mirror is subjected to feedbackcontrol to 3,433,959 3/1969 Atwood et a1. ..250/199 reduce 10w frequencyVariations i length f the u cavity 2,948,152 8/1960 Meyer ....356/106 X3,170,122 2/1965 Bennett ..33 1/945 Claims, 6 Drawing lF/ignrcs PATENTEBJAN18i972 3.635.562

SHEET 1 BF 3 FIG. 1

INVENTOR JEAN-MICHEL CATHERIN ATTORNEYS BY S FMM $31 FAT ENT EIJ m 1 21912 FIG. 3

OlPTlICAlL llNTERFEROMETER FOR DETECTION OF SMALL DTSPLACEMENTSBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to the detection and measurement of smalldisplacements by optical interferometry, and more particularly, but notexclusively, to microphones having very high sensitivity when thesedisplacements are of vibratory nature.

2. Description of the Prior Art It is known to measure distances bymeans of multiple-wave optical interferometers, such as cavities of theFabry-Perot type, which consist of two parallel semireflecting mirrors.The measurement error may be made very much smaller than the wavelengthof the monochromatic light employed. With regard more particularly tothe detection of small displacements which may be of a vibratory nature,French Pat. No. 1,484,71 1, taken out on the 7th Apr. 1964, by thepresent applicants, describes a laser situated between two mirrors, oneof which is firmly attached to a member which receives the vibrations tobe detected. The small displacements of this mirror produce proportionalvariations of the frequency of the light wave emitted by the said laser,which variations are detected by causing this wave to beat with anotherlight wave emitted by an auxiliary reference laser. The device alsocomprises a loop for servocontrol of the mean position of the vibratingmirror with the aid of a transducer. To the transducer are transmittedonly those components of the output signal which are of much lowerfrequency than the displacements detected, accomplished by means of afrequency discriminator and other elements such as, for example, alow-pass filter. It is sometimes troublesome thus to disturb theoperation of a laser, the adjustment of which is always a delicatematter.

SUMMARY OF THE INVENTION The present invention accordingly relatesparticularly to a device for detecting small displacements, including asource of monochromatic light cooperating with an assembly of twoparallel mirrors forming a resonating cavity of the F abry-Perot type.One of the mirrors is fixed and the other is suspended by a flexiblesuspension and undergoes the displacements. A detector is arranged forthe light issuing from the cavity, the intensity of which light dependsupon the distance L between the said two mirrors. The device isdistinguished particularly by the fact that the said cavity is separatedfrom the said source and by a transducer which influences the distance Dbetween the nonsuspended mirror and the said suspension.

The transducer is controlled by the detector in such manner as toproduce a followup tending to lessen the variations of the said distanceL which are produced by the said displacements, regardless of thefrequency of these displacements.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a general diagram of thedevice according to the invention;

FIG. 2 is a diametral sectional view of the F abry-Perot cavity of thesaid device, which cavity will hereinafter be simply referred to as theFF. cavity;

FIGS. 3 to 5 are sectional views of the same cavity employed inmicrophony, and

FIG. 6 is a general view of the optical device according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates an F .P.cavity which receives a portion 2 of a beam 3 emitted by a laser 4. Theother portion 5 of the beam 3 is deflected by a semitransparent strip 6on to a photosensitive cell 7.

If the luminous power emitted by the laser 4 is I;,, the luminous powersI and I, of the up 2 and 5 are proportional to the coefficients ofreflection and of transmission of the strip 6, which are constant for agiven position of the strip.

The beam 2 entering the cavity ll undergoes therein a series ofreflections on the semitransparent mirrors Ill and 2 whereby there arefinally set up a transmitted" beam MD and a reflected" beam it)", whichis thereafter again reflected on the strip 65.

The mirror 9 is mounted on a flexible suspension llll, which enables itto move parallel to itself. It is to the said mirror 9 that thedisplacements to be detected are applied, thesedisplacements onlycausing in principle variations of the distance L between the mirrors 8and 9 which are very much less than one quarter of the wavelength of thelight emitted by the laser 4. When such a variation occurs, the power 1of the transmitted beam lltl also undergoes a variation, which isdetected by the photosensitive cell 12 which receives the beam W. it isobvious that the cell 112 could just as well receive the reflected beamMD, the power of which undergoes an equivalent variation. Indeed, one ofthe properties of the FF. cavity is that all the power introduced in thecavity (disregarding the losses due to absorption in the ambient mediumand in the mirrors and 9) appears at the output resolved into two parts,one of which is transmitted" and the other reflected, the sum of thesetwo energies 1 and I therefore necessarily remaining approximately equalto the constant input power [2.

The use of the reflected beam 10 instead of the transmitted beam 10' isadvantageous if the device according to the invention is employed as amicrophone, because it is then desirable that the obstacle constitutedby the cell t2 should not be positioned in the path of the sound wavestravelling towards the mirror 9.

The signals supplied by the cells 7 and T2 are amplified at 13 and 14and applied to a differential amplifier 15 The gains of these amplifiersare so chosen that, in the absence of displacement of the movable:mirror 9, the output signal of the device according to the invention iszero. This differential arrangement, which comprises on the one hand ameasuring circuit consisting of the cell 7 and the amplifier 13, thenpermits compensation for certain errors (deviations), the cells 7 and12, which are identical, receiving the same light under like conditions.More particularly, the balance is not affected by a variation of thepower of the laser 4. Under these conditions, in the presence of adisplacement of the mirror 9, the output signal is proportional thereto.For this purpose, it is also necessary to choose a position in theneighborhood of an appropriate point of the curve giving the variationof the power l issuing from the cavity as a function of the distance Lbetween the mirrors 8 and 9, so as to describe only a portion of thiscurve which is comparable to a section of a straight line having a slopedifferent from zero, which means that the variations of L are very smalland limits a priori the dynamic range of the device.

It is known that the curve in question is not monotonous, since itconsists of a regular series of peaks spaced apart by a half of awavelength. It has already been stated that the variations of thedistance L are small in relation to one quarter of the wavelength of thelight employed. In the frequent case in which the displacements of themirror 9 are greater than one quarter of this wavelength, the variationsof L are reduced below this value, in accordance with an importantfeature of the invention, by means of a followup loop. The lattercomprises, in addition to the members already mentioned, a transducerwhich influences the distance between the mirror h and the diaphragmIll, and which is controlled by the output signal.

In the described example, the said transducer, which may be of any knowntype, notably of the magnetostrictive type, is a piezoelectric ceramicelement 116 which connects the mirror ll to the diaphragm l1 and whichis metallized on both faces, the external metallization receiving theoutput signal. The latter thus controls the distance L, which in tumdetermines the intensity l of the transmitted beam and the outputsignals of the cell 12 and amplifiers 14 and 115, the whole assemblyconstituting the aforesaid followup loop, which tends to maintainconstant the distance L. The adjustment of the mean value of thedistance L is obtained by means of the same followup loop and resultsfrom the relative values of the gains of the amplifiers l3 and 14, whichare so chosen that the output signal of the amplifier 15 brings thedistance L to the desired value in the absence of any detectabledisplacement.

Obviously, this value is so chosen that the followup is stable. Thepossibility of this adjustment is symbolically represented by thepotentiometer 13', which acts on the gain of the amplifier 13. Such afollowup adjustment has the advantage of not being destroyed byaccidental variations of L, for example of thermal origin. Naturally,the nonzero inoperative or bias value of the output signal resultingtherefrom makes it necessary to calculate the utilizable output signal,which is proportional to the measured displacement, from this value.

An important advantage due to the followup control which has beendescribed is the possibility of making a microphone very sensitive notonly in the frequency range assigned thereto by the characteristics ofinertia, rigidity, etc., of its movable equipment (diaphragm 11 andmirror 9), but also in much wider and readily adjustable ranges. Forthis purpose, the points of attachment of the cavity 1, which areindicated by arrows in FIGS. 3, 4 and 5, are adjusted.

FIG. 3 corresponds to the case of high acoustic frequencies. When a waveimpinges upon the mirror 9, the latter tends to move, for example to beshifted to the left during the compres sion phase of thiswave. Since thecavity 1 is fixed at its upper end, the previously described followuploop, which tends to maintain the distance between the two mirrors,causes the free end of the ceramic element 16 to move towards the right,which tends to minimize the displacement of the mirror 9 by way of thediaphragm 11, and thus reduces its apparent inertia. Since the mass ofthis mirror has been made small, this results in a considerableimprovement in the possibilities of high-frequency response of themicrophone thus constructed. It is to be noted that in this case thecavity is not mounted in fluidtight manner but is open to theatmosphere, since the internal pressure of the cavity must remainapproximately constant. One opening 22 is shown in FIG. 3.

FIG. 4 corresponds to the case of low acoustic frequencies. The ceramicelement 16 is fixed at its end on which the diaphragm 11 is mounted. Inthis case, the latter is advantageously mounted on a support 17 whichencroaches upon the internal volume of the cavity 1 so as to minimizethe volume displaced by a movement of the mirror 9. When the lattermoves to the left under the effect of the incident acoustic wave, thefollowup shifts the mirror 8 by an almost equal distance in the samedirection. Owing to the difference between the volumes displaced by themovement of the mirrors 8 and 9, there is set up in the cavity 1, whichis closed or optionally formed with acoustic damping slots, a depressionwhich tends to accentuate the displacement of the mirror 9 to the left,which is opposed by the rigidity of the diaphragm 11. This results in areduction of the apparent rigidity of the said diaphragm, whereby thepossibilities of response to low frequencies are enhanced.

FIG. 5 corresponds to intermediate frequencies, the point of attachmentof the ceramic element 16 then being chosen on the length of the latteras a function of the frequency range under consideration, on the linesof the arrangement of FIG. 3 or of that of FIG. 4, depending uponwhether higher or lower frequencies are concerned.

The device according to the invention may also constitute anaccelerometer which measures the acceleration along the commonperpendicular to the mirrors 8 and 9, by the displacement of thesuspended mass consisting of the mirror 9, to which there may beattached if necessary an auxiliary mass, and of the resilient suspensionmeans.

The construction of the device for the detection of small displacementsaccording to the invention may comprise, for example (see FIG. 6), alaser 4 of the helium-neon type which gives a light of a wavelength of6,328 A. with an output energy of several milliwatts. The F .P. cavity 1is formed of two mirrors having multidielectric layers, only the mirror9 being shown in the figure. These mirrors are separated by a cylinder16 consisting of piezoelectric ceramics, having a diameter of the orderof a centimeter and a length of a few centimeters. This cylinder iscentered and maintained by locking screws such as 20 screwed into afixed ring 18 and adjustable by means of their heads, such as 19,whereby it is possible to displace the said cylinder in relation to thesaid ring as a function of the frequency range to be detected.

The optical elements, such as the laser tube 4, the cavity 16, the cell12 and the semireflecting strip 6 may be mounted on an optical bench 21,for example of marble, and preferably on an antivibration table shieldedfrom atmospheric disturbances.

The signal collected at the output of the differential amplifier may betreated by the methods currently employed by the person skilled in theart.

It is obvious that some embodiments of the present invention have beendescribed by way of nonlimiting illustration and that any modificationsof detail may be made therein without departing from its scope orcertain means may be replaced by equivalent means. It is notablypossible to effect the adjustment of the distance L by applying to theceramic element 16 a unidirectional voltage obtained from an auxiliarycircuit which is separate from the followup loop which has beendescribed. It is also possible, in order to facilitate the processing ofthe electric signals, to modulate the intensity of the luminous beamemanating from the laser 4, whereby an alternating output signal at themodulation frequency of the light may be obtained. It may be desirablefor the said frequency to be very much higher than the frequency of thedisplacements to be detected and to control the ceramic element 16 onlythrough a rectifying and filtering unit which suppresses this highmodulation frequency and leaves only the signal corresponding to thedetected displacements.

While the invention has been described with reference to preferredembodiments thereof, it is to be understood that certain changes andmodifications may be made by those skilled in the art without departingfrom the spirit and scope of the invention, except as limited by theappended claims.

What is claimed is:

1. An apparatus for measuring small movements comprismg:

a. a resonant Fabry-Perot cavity including first and second parallelpartially reflecting mirrors, said mirrors positioned to be traversed bya beam of monochromatic light from a source directing light into saidcavity;

b. flexible suspension means for suspending said first mirror such thatit is movable in a direction along the axis perpendicular to the planeof said second mirror, the movement of said first mirror caused by someexternal excitation;

c. a control means for fixedly supporting said second mirror and forcontrolling the distance between said second mirror and said flexiblesuspension means;

d. a photosensitive detector means positioned at the output of saidcavity for receiving the light output of said cavity and for producingan electrical output signal in response thereto indicative of themovement of said first mirror; and

e. a feedback loop means, having an input connected to the output ofsaid photodetector means and having an output connected to said controlmeans, for Supplying a signal to said control means in accordance withthe output of said photodetector means, whereby the distance betweensaid second mirror and said flexible suspension means tends to bemaintained substantially constant.

2. The apparatus of claim 1 wherein said flexible suspension meanscomprises a membrane, said first mirror suspended in the center thereof.

3. The apparatus of claim 1 wherein said control means comprises apiezoelectric ceramic tube with said second mirror fixedly attached toone end thereof and said flexible suspension means attached to the otherend thereof.

4. The apparatus of claim 3 further comprising means attached to saidceramic tube in the plane of said second mirror for causing the end ofsaid ceramic tube attached to said suspension means to move in adirection perpendicular to the plane of said second mirror.

5. The apparatus of claim 4 further comprising means for keeping theinterior of said cavity at a constant pressure.

6. The apparatus of claim 3 wherein the surface area of said membraneand said first mirror exposed to said cavity is less than the surfacearea of said second mirror.

'7. The apparatus of claim 3 further comprising means attached to saidceramic tube at the midpoint thereof for limiting the movement of saidceramic tube in a direction perpendicular to the plane of said secondmirror.

b. An apparatus for measuring small movements comprismg:

a. a monochromatic light beam source;

b. a resonant Fabry-Perot cavity including first and second parallelpartially reflecting mirrors, said mirrors positioned to be traversed bya first portion of said monochromatic light beam;

c. flexible suspension means for suspending said first mirror such thatit is movable in a direction along the axis perpendicular to the planeof said second mirror, the movement of said first mirror caused by someexternal excitation;

. a control means for fixedly supporting said second mirror and forcontrolling the distance between said second mirror and said flexiblesuspension means;

. a first photosensitive detector means positioned at the output of saidcavity for receiving the light output of said cavity;

. a deflector means for deflecting a second portion of saidmonochromatic light beam; a second photosensitive detector means fordetecting said second portion of said monochromatic light beam;

a comparator means for comparing the output of said first

1. An apparatus for measuring small movementS comprising: a. a resonantFabry-Perot cavity including first and second parallel partiallyreflecting mirrors, said mirrors positioned to be traversed by a beam ofmonochromatic light from a source directing light into said cavity; b.flexible suspension means for suspending said first mirror such that itis movable in a direction along the axis perpendicular to the plane ofsaid second mirror, the movement of said first mirror caused by someexternal excitation; c. a control means for fixedly supporting saidsecond mirror and for controlling the distance between said secondmirror and said flexible suspension means; d. a photosensitive detectormeans positioned at the output of said cavity for receiving the lightoutput of said cavity and for producing an electrical output signal inresponse thereto indicative of the movement of said first mirror; and e.a feedback loop means, having an input connected to the output of saidphotodetector means and having an output connected to said controlmeans, for supplying a signal to said control means in accordance withthe output of said photodetector means, whereby the distance betweensaid second mirror and said flexible suspension means tends to bemaintained substantially constant.
 2. The apparatus of claim 1 whereinsaid flexible suspension means comprises a membrane, said first mirrorsuspended in the center thereof.
 3. The apparatus of claim 1 whereinsaid control means comprises a piezoelectric ceramic tube with saidsecond mirror fixedly attached to one end thereof and said flexiblesuspension means attached to the other end thereof.
 4. The apparatus ofclaim 3 further comprising means attached to said ceramic tube in theplane of said second mirror for causing the end of said ceramic tubeattached to said suspension means to move in a direction perpendicularto the plane of said second mirror.
 5. The apparatus of claim 4 furthercomprising means for keeping the interior of said cavity at a constantpressure.
 6. The apparatus of claim 3 wherein the surface area of saidmembrane and said first mirror exposed to said cavity is less than thesurface area of said second mirror.
 7. The apparatus of claim 3 furthercomprising means attached to said ceramic tube at the midpoint thereoffor limiting the movement of said ceramic tube in a directionperpendicular to the plane of said second mirror.
 8. An apparatus formeasuring small movements comprising: a. a monochromatic light beamsource; b. a resonant Fabry-Perot cavity including first and secondparallel partially reflecting mirrors, said mirrors positioned to betraversed by a first portion of said monochromatic light beam; c.flexible suspension means for suspending said first mirror such that itis movable in a direction along the axis perpendicular to the plane ofsaid second mirror, the movement of said first mirror caused by someexternal excitation; d. a control means for fixedly supporting saidsecond mirror and for controlling the distance between said secondmirror and said flexible suspension means; e. a first photosensitivedetector means positioned at the output of said cavity for receiving thelight output of said cavity; f. a deflector means for deflecting asecond portion of said monochromatic light beam; g. a secondphotosensitive detector means for detecting said second portion of saidmonochromatic light beam; h. a comparator means for comparing the outputof said first and second photosensitive detector means to provide anoutput signal indicative of the movement of said first mirror; and i. afeedback loop means having an input connected to said comparator meansand having an output connected to said control means, for supplying asignal to said control means in accordance with the output of saidcomparator whereby the distance between said second mirror and saidflexible suspension means tends to be maintained substantially constant.